BIP 11/30: Migrating welcome emails

Build in public day 11/30: I'm moving the welcome email series over to email octopus and syncing attributes to show things like a dynamic user habit page.

The benefit is having more email analytics than what we got sending it from Laravel, so we don't have to waste time reinventing the wheel.

With habits the important thing is getting new users to a 7 day streak, so I'll write the rest of the welcome series to help do that and the analytics will help me understand what's working over time.